This invention relates to an optical network which connects each of optical transmitters to each optical receiver through an optical switch and, in particular, to a method of controlling the receivers and the optical switch.
Heretofore, it is known in the art that an optical spatial switch in an optical network is large in capacity and small in size as compared with an electrical switch. Such an optical spatial switch has been proposed together with an optical network in Japanese Unexamined Patent Publication No. Hei 3-207139, namely, 207139/1991.
Specifically, a switching operation is carried out by the optical switch to switch optical signals sent out from an optical transmitter which includes an E/O (electro-optic) converter. The switched signals are sent to an optical receiver through the optical switch. The optical receiver converts the switched signals into electric signals by an O/E (opto-electric) converter of the optical receiver and amplifies the electric signals into an amplified electric signal by an amplifier of the optical receiver.
In the optical network using the spatial optical switch, optical power which is given to the optical receiver is inevitably reduced due to an optical branch or an optical coupler. This results in a reduction of the scale of the optical switch, or of throughput per port of the optical switch. In order to improve the throughput, a WDM (Wavelength Division Multiplex) technique has been often used. However, the optical switch becomes large in size and is subjected to a restriction of the number of wavelengths used therein when the WDM technique is adopted.
On the other hand, it is a recent trend that an erbium-doped (Er-doped) fiber amplifier (EDFA) has been used in each optical receiver as an optical preamplifier to be optically coupled to the optical switch. In this event, however, it often happens that optical receiving elements, such as photo-diodes, are broken or operations become unstable in rear stage circuits connected after the EDFA.
Herein, it is to be noted that each of the optical receivers is not always continuously supplied with a sequence of input optical signals from an optical transmitter. In other words, the optical receiver is often intermittently given the input optical signals from the optical transmitter with a time interval kept between two adjacent ones of the input optical signals. It often happens that such a time interval between the two adjacent input optical signals lasts for more than several milliseconds. This shows that the optical receiver is put into an inactive or quiescent state during the time interval between the two adjacent input optical signals which may be called an anterior optical signal and a posterior optical signal hereinafter.
Under the circumstances, it has been found out that the above-mentioned breakage of the optical receiving elements, the rear stage circuits, and the like is often caused to occur when the optical receiver has the optical preamplifier formed by the EDFA and when an output optical signal which accompanies an optical surge when the posterior optical signal is supplied with the optical receiver a predetermined time interval, such as several milliseconds, after reception of the anterior optical signal. As a result, the output optical signal which corresponds to the posterior optical signal has a great amplitude at a leading part thereof, which brings about the breakage of the optical receiving elements and so.
It is therefore an object of this invention to provide an optical network which is capable of avoiding occurrence of an optical surge in an optical receiver and, as a result, preventing breakage of optical receiving elements connected to the optical receiver.
It is another object of this invention to provide a method which is capable of operating the optical receiver without occurrence of the optical surge.
It is still another object of this invention to provide a method which is capable of mitigating a restriction of a scale or a restriction of the throughput.
A method to which the present invention is applicable is for use in controlling an optical network comprising a specific optical receiver which is operable in a sequence of optical input signals with a time interval left between two adjacent ones of the optical input signals to produce a sequence of optical output signals. According to an aspect of the present invention, the method comprises the steps of sending an additional sequence of optical signals to said optical receiver in addition to the optical input signal sequence and combining the additional optical signal sequence and the optical input signal sequence into a sequence of combined optical signals so that each of the combined optical signals is interposed between the two adjacent optical input signals. The additional sequence of the optical signals may be either a sequence of optical input signals directed to the additional optical receiver different from the specific optical receiver or a sequence of dummy optical signals generated by a dummy signal generator.
The method further comprises the step of discriminating the optical input signal sequence from the combined optical signal sequence after the combined optical signal sequence is allowed to pass through the specific optical receiver.
According to another aspect of the present invention, a method is for use in controlling a switching operation of an optical network comprising a plurality of optical transmitters, a plurality of optical receivers each of which includes an optical preamplifier, an optical switch between the optical transmitters and the optical receivers by using optical signals, and a switch controller controlling of the optical switch. The method comprises the step of connecting each of the optical receivers to the plurality of the optical transmitters by carrying out the switching operation within a predetermined time interval under control of the switch controller.
According to still another aspect of the present invention, an optical network is for use in connection between an optical transmitter and an optical receiver. The optical network further comprises at least one of dummy optical signal generators that generates a sequence of dummy optical signals, an optical switch which connects the dummy optical signal generator to said optical receiver, and a switch controller which controls the optical switch to connect the optical receiver to both the optical transmitter and the dummy optical signal generators.